Engine loses (electrical) power over bumps
 

93 Ranger, 4 cyl, engine hesitates (loses power) going over bumps. Check battery, check alternator, check connections. I find that I can reproduce the problem banging on the power distribution box. I open it up, check all terminals on the fuses and relays and diode in there. All looks good. Tapping the box still causes problems. Take it off, look at the wires entering, but I don't see any of the corrosion there, but wouldn't unless I start pulling terminals out one by one.

Any idea what I should look to do next? I sprayed contact cleaner in the box with everything pulled, put it back together ran fine for a short period. Today into work, the battery gauge starts fluctuating, back and forth, then drops to 8 volt and sticks. I get to work OK , but my battery is dead from running the lights and blower without being recharged. I jump it, it starts fine, is charging, test the alternator leads, they are giving out proper voltage (around 15 volts), battery passes test.

I'm not sure if it's possible to replace the power box under the hood, it looks like it would be a pain.

Any ideas?